Tracing volatile cycling from subduction to outgassing along the Aleutian Arc
Abstract
Volatile cycling drives volcanism in subduction zone settings. Volatiles can originate from the subducted slab, mantle wedge and/or crust. Each region has characteristic isotopic signatures, which can be used to determine volatile provenance. Carbon (C) and Helium (He) isotopes in particular are useful for distinguishing volatile sources. Here we combine new constraints on the isotopic composition of subducted sediment inputs and volatile outputs to trace volatile cycling within the Aleutian-Alaska Arc.
Distinct changes in C isotopes from volcanic gas emissions are seen along the Aleutian-Alaska Arc, with isotopically light C (<-10‰ δ13C) being predominant in the Central Aleutians, compared to a heavier C signature seen in the East and West, presumably due to a greater mantle or carbonate contribution. Helium isotopes in both fumaroles and crushed olivines indicate mantle-like helium emissions across the arc. Along-arc variations in the C isotopic composition and flux of subducted sediments are also observed, with δ13C of subducted sediments ranging from -7 to -19‰ and higher sediment fluxes subducted in the trench outboard of the Eastern and Central arc segments, compared to low sediment fluxes subducted in the West. We use a two-component mixing model to calculate the expected C isotopic composition of volcanic gas outputs across the arc, assuming a mixture of subducted sediment and mantle-derived volatile sources. We find that this simple mixing model reproduces well the volcanic gas C isotopic composition for the Central Aleutians, but not the Eastern and Western arc segments, which show heavier C signatures. We attribute these discrepancies to additional C sources not considered in our model. In particular, crustal C may be contributing to volcanic gas emissions in the Eastern sector (Alaska Peninsula), which is built on continental crust, while the Western Aleutian gas C composition could be explained by a greater proportion of mantle C and/or additional C from melting of the subducted slab (expected to be carbonate-dominated and therefore isotopically heavy, > 0 δ13C). Here we present new mixing model results that incorporate slab melting to test our hypotheses and elucidate volatile cycling within the Aleutian arc.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V43I0246L
- Keywords:
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- 1030 Geochemical cycles;
- GEOCHEMISTRYDE: 4930 Greenhouse gases;
- PALEOCEANOGRAPHYDE: 8185 Volcanic arcs;
- TECTONOPHYSICSDE: 8408 Volcano/climate interactions;
- VOLCANOLOGYDE: 8412 Reactions and phase equilibria;
- VOLCANOLOGYDE: 8419 Volcano monitoring;
- VOLCANOLOGYDE: 8430 Volcanic gases;
- VOLCANOLOGY